The concept and practice of chemical beneficiation process comes from the limitation of physical beneficiation processes. Broadly, a chemical beneficiation process is possible by chemical leaching of mineral matter present in coal or, dissolving organic matter of coal in various organic solvents. As coal is heterogeneous mixture of organic and inorganic constituents, solvolysis of coal varies with its composition, maturity, and structural characteristics. Since the mineral matter (non-combustible) available in specific geographical locations is very finely disseminated in the organic mass, it is really very difficult to remove the non-combustible mineral matter by conventional physical coal washing techniques. Presence of high percentage of near gravity material in such type of coal makes the scope of implementing a gravity separation process limited. This indicates that chemical treatment may be the right approach to overcome the limitation of physical beneficiation methods. A plurality of technical literature is available on chemical beneficiation techniques that employ highly corrosive chemicals (mostly acids and alkalis). Recovery or regeneration of these chemicals is very important to make this technology viable. A parallel approach towards lowering ash could be through recovering the premium organic matter from coal by solvent refining. Literature reveals that most of the research work on this subject was carried out with an objective to produce ultra clean coal or super clear coal with ash content less than 0.2% for high-tech end uses. This conventional solvent refining process does not serve the objective of low ash coal requirement of steel industries mainly, because of low recovery which makes the process uneconomic especially when such an ultraclean coal is not absolutely desired, and the cost of the process is too high as entailed by low yields.
The main advantages of the prior art process are i) ease of recovery of solvent in the main process steam, ii) solvolytic efficiency of recovered solvents as that of fresh solvent, iii) 95-98% recovery of the solvent, iv) improved coking properties of clean coal, and v) availability of industrial organic solvents.
According to the existing process, coal, solvent (N-Methyl-2-Pyrrolidone, NMP) and co-solvent (Ethylenediamine, EDA or Monoethanolamine, MEA) are mixed thoroughly to produce coal slurry. The coal slurry is extracted in a known manner which includes coal-solvent mixture. The mixture is separated in a separation unit to produce a coarser fraction and a finer fraction. The finer fraction is fed to an evaporator unit to allow 70 to 80% of solvent recovery. The hot concentrated coal-solvent mixture is then flushed in a precipitation tank to precipitate the coal, where, water as an anti-solvent is being used. Water separates the solvent from coal and we get water-solvent mixture, which is fed to distillation unit to separate solvent and anti-solvent. And precipitated coal is separated in a filtration unit.
In the existing processes pretreatment of coal is not being done. It is a solvent extraction of coal process without any pretreatment of coal. So there is a scope of pretreatment of coal by microwave and ultrasonication, (1) to open up the pores of coal samples, facilitation and (2) disrupt the coal (organic matter) and mineral matter physical (or chemical) association, for enhancement of clean coal extraction. Our aim is to have comparative study of solvent extraction with pretreatment followed by solvent extraction. Also, to study room temperature dissolution as another attempt to reduce the heat consumption.
JP2001026791 discloses a method for producing an ash-less coal is to bring a raw material coal in contact with N-methyl-2-pyrrolidinone solvent or a mixed solvent of carbon disulfide with N-methyl-2-pyrrolidinone in the presence of a chlorine or fluorine compound for extracting the ash-less coal.
JP2010023018 describes a pretreatment method and device. Accordingly, scavenger, a bubbling agent, and air are fed into the slurried coal ash composed of the unburned carbon-containing coal ash and water so as to produce bubbles, and the unburned carbon is caught in the bubbles. Before the slurried coal ash is fed to a floatation apparatus for separating tail ash having a low unburned carbon content, ultrasonic waves are emitted thereto by an ultrasonic wave generating apparatus so that the separation efficiency of the unburned carbon from the coal ash is increased.
WO2010029563 teaches an improved organo-refining process to produce low ash clean coal from high ash coal, the method comprising the steps of: mixing of coal, solvent and a co-solvent; feeding the slurry to a reactor by pumping; the reactor maintaining a temperature about 200 deg. C to 300 deg. C and a pressure of 1.5 atm.; extracting coal-solvent mixture; feeding the extracted coal to a flasher unit; recovering about 30% of the solvent from the flashing unit; feeding the remaining heavy material with some coal extracts to an evaporator; extracting about 60% of solvent from the evaporator; discharging the residue from the evaporator to a precipitator; filtering the slurry in a rotary drum; collecting the super clean coal as residue containing 0.1 to 10% ash; feeding the filtrate into a distillation unit; separating water and organic material in order to recover at least 7 to 8% of the remaining solvent.
US2005236403 discloses pre-treatment of multi-phase materials. The material having a first phase of material and a second phase of material. The method comprises heating the material electromagnetically, preferably with microwaves, to produce a power density of at least 10<9>Wm<−3> in a continuous process in which the material moves into and through an electromagnetic, preferably microwave, treatment area. The material experiences exposure to microwaves, in the treatment area for a time of the order of ½ second or less before the material is passed out of the treatment area for subsequent operation.
However, the present inventors in course of developing the disclosed invention have identified that recovery of clean coal and its ash content depend on various operating parameters and feed characteristics for example, faster and convenient filtration of refluxed solution at different size, selection of solvent and co-solvent, coal and source (feed characteristics), particle size, coal-solvent ratio, extraction time, and extraction temperature.
By way of reference, prior Indian patent application numbers 1336/KOL/2008, 1088/KOL/07 and 1292/KOL/06 are incorporated herein.